1. Field of the Invention
This invention relates to a display apparatus and a driving method for a display apparatus, and more particularly to a display apparatus wherein a plurality of pixel circuits each including an electro-optical element are disposed in a matrix and a driving method for the display apparatus.
2. Description of the Related Art
In recent years, in the field of image display apparatus, an organic EL (electroluminescence) display apparatus has been developed and commercialized wherein a large number of pixel circuits each including an electro-optical element of the current driven type whose light emission luminance varies in response to the value of current flowing therethrough such as, for example, an organic EL element as a light emitting element of the pixel are disposed in a matrix. Since the organic EL element is a self-luminous element, the organic EL display apparatus is advantageous in that the observability of an image displayed is high, that no backlight is necessary and that the responding speed of the element is high when compared with a liquid crystal display apparatus wherein the light intensity from a light source (backlight) is controlled by a pixel circuit including a liquid crystal cell.
The organic EL display apparatus can adopt a simple (passive) matrix type or an active matrix type as a driving method therefor similarly to the liquid crystal display apparatus. However, although the display apparatus of the simple matrix type is simple in structure, it has such a problem that it is difficult to implement a display apparatus of a large size having high definition. Therefore, in recent years, efforts have been made to develop a display apparatus of the active matrix type wherein current to flow through a light emitting element is controlled by an active element provided in a pixel circuit in which the light emitting element is provided such as, for example, an insulated gate type field effect transistor (generally a thin film transistor (TFT)).
If it is possible to use, in a pixel circuit in which a thin film transistor (hereinafter referred to as “TFT”) is used as an active element, an N-channel type transistor for the TFT, then it is possible to use an amorphous silicon (a-Si) process in related art in formation of the TFT on a substrate. The use of the a-Si process makes it possible to achieve reduction of the cost of the substrate on which the TFT is to be formed.
Incidentally, the current-voltage (I-V) characteristic of an organic EL element generally deteriorates as time passes (aged deterioration). Since, in a pixel circuit in which an N-channel TFT is used, the organic EL element is connected to the source side of a transistor (hereinafter referred to as “driving transistor”) for driving the organic EL element with current, if the I-V characteristic of the organic EL element undergoes aged deterioration, then the gate-source voltage Vgs of the driving transistor changes. As a result, also the light emission luminance of the organic EL element changes.
This is described more particularly. The source potential of the driving transistor depends upon the operation point of the driving transistor and the organic EL element. If the I-V characteristic of the organic EL element deteriorates, then the operation point of the driving transistor and the organic EL element varies, and consequently, even if the same voltage is applied to the gate of the driving transistor, the source potential of the driving transistor changes. Consequently, the source-gate voltage Vgs of the driving transistor changes and the value of current flowing through the driving transistor changes. As a result, also the value of current flowing through the organic EL element changes, resulting in change of the light emission luminance of the organic EL element.
Further, in a pixel circuit which uses a polycrystalline TFT, the threshold voltage Vth of the driving transistor exhibits aged deterioration or differs among different pixels (individual transistors disperse in characteristic) in addition to the aged deterioration of the I-V characteristic of the organic EL element. Since, if the threshold voltage Vth is different among different driving transistors, then the values of current flowing through the driving transistors exhibit dispersion, even if the same voltage is applied to the gate of the driving transistors, the organic EL elements emit light in different luminance, resulting in loss of the uniformity of the screen.
In the past, in order to keep the light emission luminance of the organic EL element fixed without being influenced by aged deterioration of the I-V characteristic of the organic EL element or by aged deterioration of the threshold voltage Vth of the driving transistor even if such aged deterioration or change occurs, a compensation function against the characteristic variation of the organic EL element and a compensation function against the variation of the threshold voltage Vth of the driving transistor are provided for each pixel circuit. The configuration just described is disclosed, for example, in Japanese Patent Laid-Open No. 2004-361640.